Time integration of unsteady nonhydrostatic equations with dual time stepping and multigrid methods

Elsevier 2018 Journal of computational physics Vol.374 No.-

<P><B>Abstract</B></P> <P>A time integration method based on dual time stepping coupled with a multigrid and implicit iterative solver is discussed for overcoming the temporal stiffness associated with modeling unsteady atmospheric flows. Adopting the dual time stepping allows solving an unsteady flow problem as a steady-state one by introducing a pseudo-time derivative into the governing equations. Consequently, the model uses a large time step and leads to a reduction in computational time. In the framework of the dual time stepping, a full approximation storage multigrid and lower–upper symmetric Gauss–Seidel implicit solver are employed to accelerate convergence in a structured, quadrilateral grid. The discretization of a physical-time derivative is based on a second-order backward differentiation formula or a family of singly diagonally implicit Runge–Kutta methods. The spatial terms of nonhydrostatic equations are discretized with a second-order Roe scheme in the finite volume method. To assess the performance of the model in terms of accuracy and efficiency, which are indicated by the normalized error norms and wall-clock time, respectively, three idealized tests are performed with the unsteady flow problems of isentropic vortex, rising thermal bubble and inertia-gravity waves. The results of numerical experiments consistently demonstrate that the dual time stepping coupled with explicit-first, singly diagonally implicit Runge–Kutta and multigrid methods is more efficient than the three-stage, third-order explicit Runge–Kutta method with comparable accuracy. The numerical experiment also indicates that the dual time stepping is more efficient when a stretched grid is employed. The efficiency in the framework of the dual time stepping is dependent on various numerical factors, including a multigrid level and cycle, Butcher coefficients, an implicit solver and a physical-time step.</P>

Time–Frequency-Based Insulation Diagnostic Technique of High-Temperature Superconducting Cable Systems

Geon Seok Lee,Gu-Young Kwon,Su Sik Bang,Yeong Ho Lee,Seung Jin Chang,Song-Ho Sohn,Kijun Park,Yong-June Shin Institute of Electrical and Electronics Engineers 2016 IEEE transactions on applied superconductivity Vol.26 No.4

<P>For the electrical insulation of a high-temperature superconducting (HTS) cable, wrapped polypropylene laminated paper (PPLP) tape is typically used. Unfortunately, it is possible that unexpected faults at insulation layers will be present in the cables as a result of either a problematic manufacturing process or an incomplete installation procedure. In order to protect against operational failures of grid-connected HTS cable systems, this paper proposes a nondestructive diagnostic technique, i.e., time-frequency domain reflectometry (TFDR), and focuses on the characteristic of HTS cable that caused the local insulation defects. To verify the performance of the proposed method, detection and localization of local insulation failure via TFDR are compared with traditional time-domain reflectometry. The experiments are conducted at room temperature and under liquid nitrogen in order to check the efficacy of the proposed method in varieties of HTS cable's conditions. In addition, to improve the accuracy of detection and localization, a methodology to analyze incident signals, which are composed of upchirp and downchirp signals, is presented.</P>

Time–temperature superposition for foaming kinetics of Al-alloy foams

Kim, Amkee,Tunvir, Kazi,Nahm, Seung-Hoon,Cho, Seong-Seock Elsevier 2008 Journal of materials processing technology Vol.202 No.1

<P><B>Abstract</B></P><P>Applicability of time–temperature superposition principle to the foaming kinetics of aluminum (Al)-alloy foams produced by powder metallurgical method was investigated. Foaming kinetics above melting temperatures of Al–Si–Cu–Mg foams was studied. The expansion data at various furnace temperatures were collected. Well-known superposition parameters such as Larson-Miller, Orr-Sherby-Dorn, Goldhoff-Sherby and Manson-Succop were established based on the linear iso-expansion lines in plots of log(heating time) versus furnace temperature and log(heating time) versus inverse furnace temperature. In order to study the expansion kinetics of the Al-alloy foams, the expansions were measured in terms of pore fraction using an image analyzer. Finally, the linear relationship between the porosity and the superposition parameters was established.</P>

A 5-Gb/s 2.67-mW/Gb/s Digital Clock and Data Recovery With Hybrid Dithering Using a Time-Dithered Delta–Sigma Modulator

Taeho Lee,Yong-Hun Kim,Jaehyeong Sim,Jun-Seok Park,Lee-Sup Kim IEEE 2016 IEEE transactions on very large scale integration Vol.24 No.4

<P>A digital clock and data recovery (CDR) employing a time-dithered delta-sigma modulator (TDDSM) is presented. By enabling hybrid dithering of a sampling period as well as an output bit of the TDDSM, the proposed CDR enhances the resolution of digitally controlled oscillator, removes a low-pass filter in the integral path, and reduces jitter generation. Fabricated in a 65-nm CMOS process, the proposed CDR operates at 5-Gb/s data rate with BER < 10(-12) for PRBS 31. The CDR consumes 13.32 mW at 5 Gb/s and achieves 2.14 and 29.7 ps of a long-term rms and peak-to-peak jitter, respectively.</P>

Extraction of Effective Mobility from nMOSFETs With Leaky Gate Dielectric Using Time Domain Reflectometry

Yonghun Kim,Young Gon Lee,Ukjin Jung,Jin Ju Kim,Minhyeok Choe,Kyong Taek Lee,Sangwoo Pae,Jongwoo Park,Byoung Hun Lee IEEE 2015 IEEE transactions on electron devices Vol.62 No.4

<P>The effective mobility (μ<SUB>eff</SUB>) of MOSFETs with ultrathin high-k gate dielectric (EOT = 0.85 nm) has been successfully extracted using a time domain reflectometry method. A ground-signal-ground-ground probe configuration is used to analyze the gate-to-channel capacitance (C<SUB>gc</SUB>), gate-to-bulk capacitance (C<SUB>gb</SUB>), and total gate capacitance (C<SUB>g</SUB>) without a complex RF test structure. Using this method, the effective mobility can be extracted even in the presence of a high gate leakage current (~30 A/cm<SUP>2</SUP>) when the conventional split capacitance-voltage method using an impedance analyzer cannot be applied.</P>

Detection of leak acoustic signal in buried gas pipe based on the time–frequency analysis

Kim, Min-Soo,Lee, Sang-Kwon Elsevier 2009 Journal of loss prevention in the process industri Vol.22 No.6

<P><B>Abstract</B></P><P>A time–frequency technique for locating leaks in buried gas distribution pipes involves the use of the cross-correlation on two measured acoustic signals on either side of a leak. This technique can be problematic for locating leaks in steel pipes, as the acoustic signals in these pipes are generally narrow-band and low frequency. The effectiveness of the time–frequency technique for detecting leaks in steel pipes was investigated experimentally in an earlier study. The object of this paper is to identify the characteristics of this dispersive acoustic wave through analysis of the cut-off frequency by using the time–frequency method experimentally and BEM (boundary element method) theoretically for the development of an experimental tool to analyze the leak signals in steel pipe. The tool is based on experimental work and theoretical formulation of wave propagation in a fluid-filled pipe. This tool uses the time–frequency method to explain some of the features of wave propagation measurements made in gas pipes. Leak noise signals are generally passed through a time–frequency filter for detection of impulse signal related leakage.</P>

Effect of inhibitory spike-timing-dependent plasticity on fast sparsely synchronized rhythms in a small-world neuronal network

Kim, Sang-Yoon,Lim, Woochang Elsevier 2018 Neural networks Vol.106 No.-

<P><B>Abstract</B></P> <P>We consider the Watts–Strogatz small-world network (SWN) consisting of inhibitory fast spiking Izhikevich interneurons. This inhibitory neuronal population has adaptive dynamic synaptic strengths governed by the inhibitory spike-timing-dependent plasticity (iSTDP). In previous works without iSTDP, fast sparsely synchronized rhythms, associated with diverse cognitive functions, were found to appear in a range of large noise intensities for fixed strong synaptic inhibition strengths. Here, we investigate the effect of iSTDP on fast sparse synchronization (FSS) by varying the noise intensity D . We employ an asymmetric anti-Hebbian time window for the iSTDP update rule [which is in contrast to the Hebbian time window for the excitatory STDP (eSTDP)]. Depending on values of D , population-averaged values of saturated synaptic inhibition strengths are potentiated [long-term potentiation (LTP)] or depressed [long-term depression (LTD)] in comparison with the initial mean value, and dispersions from the mean values of LTP/LTD are much increased when compared with the initial dispersion, independently of D . In most cases of LTD where the effect of mean LTD is dominant in comparison with the effect of dispersion, good synchronization (with higher spiking measure) is found to get better via LTD, while bad synchronization (with lower spiking measure) is found to get worse via LTP. This kind of Matthew effect in inhibitory synaptic plasticity is in contrast to that in excitatory synaptic plasticity where good (bad) synchronization gets better (worse) via LTP (LTD). Emergences of LTD and LTP of synaptic inhibition strengths are intensively investigated via a microscopic method based on the distributions of time delays between the pre- and the post-synaptic spike times. Furthermore, we also investigate the effects of network architecture on FSS by changing the rewiring probability p of the SWN in the presence of iSTDP.</P> <P><B>Highlights</B></P> <P> <UL> <LI> An inhibitory small-world network of fast spiking interneurons is considered. </LI> <LI> Effects of inhibitory spike-timing-dependent plasticity are investigated. </LI> <LI> A “Matthew” effect in inhibitory synaptic plasticity is found to occur. </LI> <LI> Emergences of LTD and LTP are investigated via a microscopic studies. </LI> <LI> Effects of network architecture on FSS are studied in the presence of iSTDP. </LI> </UL> </P>

Circadian variations in the pharmacokinetics, tissue distribution and urinary excretion of nifedipine after a single oral administration to rats

Cao, Qing-Ri,Kim, Tae-Wan,Choi, Jun Shik,Lee, Beom-Jin John Wiley Sons, Ltd. 2005 Biopharmaceutics & drug disposition Vol.26 No.9

<P>Circadian variations in the pharmacokinetics, tissue distribution and urinary excretion of nifedipine were examined in fasted rats after administering a single oral dose at three different dosing times (08:00 am, 16:00 pm, 00:00 am). The plasma concentrations, the areas under the plasma concentration-time curve from zero to 6 h (AUC<SUB>0–6 h</SUB>) and the peak plasma concentration (C<SUB>max</SUB>) were significantly higher in the rats dosed at 08:00 am (immediately inactive), and was lower at 16:00 pm (most inactive) and 00:00 am (most active). The time to reach the C<SUB>max</SUB> (T<SUB>max</SUB>) was the shortest in the rats dosed at 08:00 am. It was very interesting to observe the double peak phenomena in the plasma concentration profiles, showing a larger peak followed by a smaller peak. There was a dosing time dependency on the tissue distribution 30 min after administration, showing a similar tendency to the pharmacokinetic behavior. However, there was no distinct dosing time dependency observed at 2 h after administration due to the extensive disposition. The cumulative urine excretion of nifedipine in the rats dosed at 08:00 am was significantly higher (about two-fold) than in those dosed at 16:00 pm and 00:00 am. The pharmacokinetics of nifedipine in the rats was consistent with that observed in human subjects in terms of the day–night clock time but the biological time was the opposite, as marked by the rest–activity cycles. These results may help to explain the circadian time-dependency of nifedipine pharmacokinetics. Copyright © 2005 John Wiley & Sons, Ltd.</P>

Further improvement of periodic control approach for relaxed stabilization condition of discrete-time Takagi–Sugeno fuzzy systems

Lee, Dong Hwan,Park, Jin Bae,Joo, Young Hoon Elsevier 2011 FUZZY SETS AND SYSTEMS Vol.174 No.1

<P><B>Abstract</B></P><P>In this paper, we present a new stabilization condition for discrete-time Takagi–Sugeno (T–S) fuzzy systems. To this end, the previously developed periodic nonparallel distributed compensation (non-PDC) control law is extended to the more general one whose gain matrix has a block lower triangular matrix form. A distinguished feature of the proposed control law is that it uses a discrete-time feedback loop incorporating information from the previous states of the system. The resultant stabilization condition is represented in the form of linear matrix inequalities (LMIs). In addition, we show that the proposed condition includes the existing one based on the periodic control approach as a particular case. Finally, examples are given to show that the proposed condition provides less conservative results than those of the existing ones published in the literature.</P>

Temperature-Responsive Tensile Actuator Based on Multi-walled Carbon Nanotube Yarn

Kim, Hyunsoo,Lee, Jae Ah,Sim, Hyeon Jun,Lima, Má,rcio D.,Baughman, Ray H.,Kim, Seon Jeong Springer Berlin Heidelberg 2016 Nano-micro letters Vol.8 No.3

<P>Many temperature indicators or sensors show color changes for materials used in food and medical fields. However, they are not helpful for a color-blind person or children who lack judgment. In this paper, we introduce simply fabricated and more useful low-temperature indicator (~30 °C) for devices that actuates using paraffin-infiltrated multi-walled carbon nanotube (MWCNT) coiled yarn. The density difference of MWCNT yarn provides large strain (~330 %) when heat causes the melted polymer to move. Furthermore, the MWCNT yarn decreases the melting point of paraffin. These properties allow control of the actuating temperature. In addition, mechanical strength was enhanced by MWCNT than previously reported temperature-responsive actuators based on shape memory polymers. This simply fabricated temperature indicator can be applied in latching devices for medical and biological fields.</P><P><B>Electronic supplementary material</B></P><P>The online version of this article (doi:10.1007/s40820-016-0084-6) contains supplementary material, which is available to authorized users.</P>